Natural gas sweetening using aqueous methyldiethanolamine (MDEA, 45-50 weight %) is currently used by e.g. GASCO Company (Habshan, Abu Dhabi) to scrub H2S/CO2. The rich MDEA solution is fed to a regeneration column where heat is applied to strip the acid gas components to get back the lean MDEA solution. In this absorption process, formate, acetate, propionates etc. as organic acid anions are formed by the reaction between aerial oxygen and H2S and CO2. The reactions between these anionic species and protonated MDEA forms heat stable salt (HSS), which cannot be removed from the solvent by heating in the regenerator. Continuous accumulation of these HSS in the MDEA solvent deteriorates its quality to lower its absorption capacity.
Another serious concern of the unit are the other major contaminants like heavy metal ions, arising mostly from corrosion and erosion of process equipment and accumulating in the lean MDEA solvent. Some heavy metal ions might come from the demineralized water added as make-up water to maintain the concentration of lean MDEA solution from time to time. Moreover, MDEA thermally degrades to organic compounds like diethanolamine (DEA), bicine etc. Therefore, regeneration of MDEA is inevitable for the industry. Thus, an adsorbent containing calcium alginate hydrogel with filler like kaolinite, quartz or activated carbon was prepared to remove those contaminants from lean MDEA using batch as well as column studies.
The conventional methods for removal of total contaminants from lean MDEA have been preceded by using the ion exchange resin, separation membrane, etc. It has been proposed to remove heat stable salts from lean alkanolamine solution by ion exchange resins. For example, at the 2014 AIChE Annual meeting, an abstract was presented (see e.g. http://www3.aiche.org/proceedings/Abstract.aspx?PaperID=367797) discussing a base polymeric material which however showed stability problems when used in trials.
Further, U.S. Pat. Nos. 2,797,188, 4,122,149, 4,170,628, 4,477,419, 4,758,311, 4,795,565, 4,970,344, 4,999,113, 5,006,258, 5,162,084, 5,277,822 and 5,788,864 etc. described the removal of heat stable salt anions removal by exchange with hydroxide ions from an anion exchange resins and cations, such as sodium and potassium, by hydrogen ion from a cation exchange resins.
However, in case of the removal using ion exchange resin or separation membrane, it is difficult and expensive to treat the polluted lean MDEA containing a lot of heat stable salt anions, heavy metal ions and thermally degraded organic compounds as contaminant. Though, removal of heavy metal ions like chromium or iron were not described anywhere. Despite the foregoing developments, there is significant room for improvement in the art.